Troubleshooting a Lab Series L5

[kvandekrol]

I have a 1980 Lab Series L5 that's been giving me a lot of trouble lately. After resoldering the preamp and power supply PCBs and replacing a couple of components that looked weak, I've brought it down to the following two problems:

1) A constant "crackle" sound. It's not a hum, hiss or popping sound, but I don't know how else to describe it. It fluctuates in volume – sometimes it's pretty loud and sometimes it's barely audible, but it's always there. The noise level stays the same regardless of the master volume, so I think this is coming from someplace in the power supply rather than the preamp. It still processes input signal just fine, but that crackling is very unnerving! Does this sound like a problem with a particular component? The next step is replacing components one by one which I'd rather not do if I can avoid it...

2) The compressor sounds like it's on its last legs. Whenever it kicks in and the compressor LED shows action, it cuts in and out very erratically until the signal decays below the compressor's threshold. This is a new problem. It sounds like it might be a bad solder joint or something, but it started happening about three weeks after I resoldered all of the joints on the preamp. It's fine if I turn the compressor switch off. Would this be fixed by replacing an opamp someplace, or is it more complicated than that?

I found a few older threads on this board about Lab Series troubleshooting, and it seems like there are a few people hanging around who know a lot about these amps. But just in case, here's a link to the schematic, board layout and service docs:

http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html

Thanks everyone!

[Enzo]

Isolate the problem.

ANy control that affects the sound of the noise is AFTER the source of the noise.  SO if a tone control affects the tone of it,then it came in before the control.  COntrols that do not affect the sound are BEFORE its source.

It sounds like it is in your power amp.  PLug a guitar into the POWER AMP IN jack and dial it down to zero volume.  If teh noise remains, then it is indeed in the power amp.  If that kils it, then the noise is coming from the preamp.

You could also connect a cord from the PREAMP OUT jack to the input of some other amp to see if the noise is on the preamp output.  In fact, do both.

If the noise is on the preamp side, look at your schematic.  Between the MV, which you report doesn't affect the noise, and the preamp out is source follower Q101, which could easily be noisy.  Chip A111 could as well.  And output cap C138 could be involved.

If the noise is in the power amp, my inclination is to suspect the input differential pair or Q304.

For crackles I usually expect a noisy semiconductor, though resistors get noisy too.  A cap is less likely - they make their own kinds of trouble.